It is known from experience that untrained personnel make operating errors especially in renewing the processing fluids and in cleaning the troughs. This work is usually carried out weekly or at longer periods in the case of limited use. First the processing fluid is let out, pumped out or poured out then the troughs are flushed with cleaning fluid which is washed out with water and then the troughs are filled with new processing fluids. A particular complication of this cleaning work is that not only the troughs but also the associated fluid circulating system must be cleaned. The fluids must be continually pumped in order to maintain the temperature and concentration constant. This is necessary in order to achieve satisfactory quality of the developed pictures. The circulation is generally effected by a pump which is frequently accommodated in the service part of the apparatus and is connected with the trough through supply and return conduits. After draining the fluid from a trough, the trough and pump system is flushed with a cleaning fluid. Then the cleaning fluid is to be very carefully removed as otherwise this will chemically disturb the freshly filled fluid. The pump system is hence to be repeatedly flushed with fresh water. Here air is frequently arised through carelessness so that the cleaning fluid is not completely removed whereby the newly filled fluid is wholly or at least to a certain percentage chemically spoiled so that it must soon be again renewed in order to obtain a satisfactory picture quality.